EXCHANGE 


EXCHANGE 
NGv'  IS 


Ufoe  mni\?erstt£  ot  Gbtcago 


THE  RELATION  BETWEEN  THE  ALPHA- 

RAY  ACTIVITIES  AND  RANGES  OF 

RADIOACTIVE  SUBSTANCES 


A  DISSERTATION 

SUBMITTED  TO  THE  FACULTY 

OF  THE  OGDEN  GRADUATE  SCHOOL  OF  SCIENCE 

IN 'CANDIDACY  FOR  THE  DEGREE  OF 

DOCTOR  OF  PHILOSOPHY 

DEPARTMENT  OF  CHEMISTRY 


BY 

EDWIN  DANIEL  LEMAN 


Private  Edition,  Distributed  By 

THE  UNIVERSITY  OF  CHICAGO  LIBRARIES 

CHICAGO,  ILLINOIS 

1918 


Ube  *Cinix>ersft£  ot  Gbfcaao 


THE  RELATION  BETWEEN  THE  ALPHA. 

RAY  ACTIVITIES  AND  RANGES  OF 

RADIOACTIVE  SUBSTANCES 


A  DISSERTATION 
SUBMITTED  TO   THE   FACULTY 

OF   THE   OGDEN   GRADUATE   SCHOOL  OF   SCIENCE 

IN    CANDIDACY  FOR   THE   DEGREE   OF 

DOCTOR   OF   PHILOSOPHY 

DEPARTMENT  OF  CHEMISTRY 


BY 

EDWIN  DANIEL  LEMAN 


Private  Edition,  Distributed  By 
THE  UNIVERSITY  OF  CHICAGO  LIBRARIES 
CHICAGO,  ILLINOIS 
1918 


Qc 

i-5 


CONTENTS 

PAGE 

INTRODUCTION i 

CHAPTER 

I.  THE  RELATION  BETWEEN  THE  a-RAY  ACTIVITIES  AND  RANGES  or 


RADIUM  AND  ITS  SHORT-LIVED  PRODUCTS 


II.  THE    RELATION   BETWEEN   a-RAY   ACTIVITIES    AND    RANGES   OF 

RADIOACTINIUM  AND  ITS  SUBSEQUENT  PRODUCTS     .     .     .,.,.•'  9 

III.  NOTES    . ,.    «^_^    '1     v    J     .     .  14 

Note  i .     The  Period  of  Radioactinium 14 

Note  2.     Determination  of  the  Range  of  Radioactinium      ...  :  *  •  .  17 

Notes.     The  Period  of  Actinium  X      .'•'•"._    ,   ..     .     .     .     .  ;.   .  18 


404734 


f  t 


INTRODUCTION 

At  the  present  time  all  previous  work  tends  to  indicate  that  the 
number  of  ions,  and  therefore  the  ionization  current  in  air  due  to  a  single 
particle  of  range  R,  is  very  closely  proportional  to  the  two- thirds  power 
of  R.1  This  can  be  represented  by  the  equation,  I  =  kR%,  where  /  is 
the  ionization  current,  R  the  range,  and  k  a  constant  for  particles  of  all 
ranges.  McCoy  and  Viol2  have  shown  that  the  relative  a-ray  activities 
in  the  thorium  series,  as  calculated  from  this  equation,  are  in  good  accord 
with  experimental  values.  The  purpose  of  this  paper  is  to  show  that  the 
relative  activities  of  radium  and  its  short-lived  products  and  of  radio- 
actinium  and  its  subsequent  products  are  also  in  good  agreement  with 
those  calculated  from  the  known  ranges.  Notes  on  the  period  and  range 
of  radioactinium  and  the  period  of  actinium  X  are  also  included.  The 
subject-matter  of  this  thesis  has  been  published  in  three  papers.3 

1  Geiger,  Proc.  Roy.  Soc.,  A,  82,  486,  1909;  Taylor,  Phil.  Mag.,  21,  371,  1911; 
McCoy,  Phys.  Rev.,  i,  393,  1913. 

2  Phil.  Mag.,  25,  333,  1913. 

3  McCoy  and  Leman,  Phys.  Zeit.,  14,  1280  (1913);  Phys.  Rev.,  4,  409  (1914); 
ibid.,  6,  184  (1915). 


CHAPTER  I 

THE  RELATION  BETWEEN  THE  d-RAY  ACTIVITIES  AND  RANGES  OF 
RADIUM  AND  ITS  SHORT-LIVED  PRODUCTS 

The  latest  available  data  on  the  radium  series1  which  are  used  in  this 
paper  are  given  in  Table  I. 

TABLE  I 

THE  RADIUM  SERIES 


Symbols 

Period 

Rays 

Ranges  (Cm.  at 
15°  C.) 

Radium  

Ra 

i  730    years 

a,  /3 

3    3O 

Emanation  

Em 

3  .  8  ^  days 

a 

4  16 

Radium  A 

Ra.A 

3  oo  minutes 

a. 

47^ 

Radium  B  

R&B 

26  .  7    minutes 

/3 

Radium  Ci 

RaCi 

19  5    minutes 

a(?)    13 

Radium  C2 

RaC2 

i  4    minutes 

/3 

Radium  C"  

RaC' 

io-6  seconds(?) 

a 

6.94 

The  accepted  scheme  of  disintegration  in  this  series,  according  to 
Fajans,2  is  as  follows: 


a, 


According  to  this  view,  the  product  formerly  called  radium  C  is 
complex:  d,  which  is  the  product  of  B,  disintegrates  in  two  ways:  the 
first  (principal)  with  the  production  of  a  /3-particle  giving  C';  the  second 
(subordinate)  with  the  expulsion  of  an  a-particle,  giving  C2.  The  prod- 
uct C'  also  gives  a-rays,  and  of  every  10,000  a-par tides  produced  by  the 
C  components  all  but  three  are  due  to  the  change  of  C'  into  Z),  the  balance 
being  due  to  the  change  of  d  into  C2.3  If  we  assume  that  these  conclu- 
sions are  essentially  correct,  we  see  that  the  effect  on  the  activity  caused 
by  a  minute  fraction  of  the  a-particles  of  radium  C,  having  a  different 
range  from  those  of  the  main  fraction,  would  be  negligibly  small.  I  shall 

1  Kolowrat,  Le  Radium',  n,  i,  1914. 

2  Phys.  Zeit.,  13,  699,  1912.  3  Fajans,  loc.  cit. 


a-RAY   ACTIVITIES   AND   RANGES   OF   RADIUM  3 

neglect  the  complexity  of  C  in  the  discussions  in  this  paper,  merely  point- 
ing out  that  the  present  work  furnishes  no  data  regarding  this  important 
question. 

The  method  of  determining  the  relative  a-ray  activities  of  Ra  and 
its  short-lived  products  consisted  in  preparing  a  Ra-BaSO4  film  free 
from  all  of  the  subsequent  products  of  Ra  and  measuring  its  initial  and 
final  activities.  The  activities  of  such  films  increased  for  about  six 
weeks  after  the  preparation  of  the  film  and  then  remained  practically  con- 
stant during  several  months  of  observation,  the  constant  activity  in  each 
case  being  taken  as  the  final.  The  Ra-BaSO4  film,  free  from  its  sub- 
sequent products,  was  prepared  in  the  following  manner:  One  c.c. 
of  a  solution  of  pure  radium  chloride  in  dilute  hydrochloric  acid  was 
diluted  to  10  c.c.;  the  solution  was  heated  to  its  boiling-point,  and  a 
current  of  air  was  bubbled  through  it  for  thirty  minutes,  the  solu- 
tion being  kept  near  its  boiling-point  and  a  few  cubic  centimeters  of 
water  being  added  from  time  to  time  to  replace  that  lost  by  evaporation. 
This  procedure  kept  the  solution  free  from  emanation,  and  allowed  Ra^l 
to  decay  practically  completely.  A  few  drops  of  lead  acetate  solution 
were  then  added  and  hydrogen  sulphide  passed  in.  The  lead  sulphide, 
which  was  precipitated,  removed  B,  C,  D,  E,  and  F.1  The  precipitate 
was  rapidly  filtered  off,  and  a  current  of  air  was  again  bubbled  through 
the  filtrate  for  ten  minutes,  keeping  it  near  its  boiling-point  and  the 
volume  constant.  Lead  sulphide  was  again  precipitated  in  the  solution, 
the  precipitate  filtered  off,  and  the  filtrate  again  treated  in  the  manner 
just  described,  making  a  total  of  three  precipitations  of  lead  sulphide 
and  removing  practically  every  trace  of  the  products  of  radium.  The 
various  operations  to  this  point  had  taken  about  fifty  minutes.  The 
filtrate  from  the  last  treatment  was  put  into  a  15  c.c.  centrifuge  tube, 
i  c.c.  of  N/ioo  barium  chloride  solution  added,  then  a  few  drops  of  dilute 
sulphuric  acid.  The  solution  was  well  shaken  and  then  centrifuged. 
The  supernatant  solution  was  decanted  from  the  precipitate,  and  the 
latter  was  well  washed,  by  decantation,  with  water  acidified  with  a  few 
drops  of  HC1,  and  finally  with  alcohol.  A  small  portion  of  this  precipi- 
tate was  spread  as  uniformly  as  possible,  with  the  aid  of  a  glass  rod  and  a 
little  alcohol,  over  a  flat  polished  brass  plate,  about  7  cm.  in  diameter. 
When  the  alcohol  had  evaporated,  the  less  firmly  adhering  particles  were 
brushed  off,  care  being  taken  to  guard  the  edges  and  back  of  the  plate 
from  radioactive  contamination.  The  films  so  prepared  were  so  thin 

1  Compare  the  work  of  McCoy  and  Viol,  loc.  cit.,  on  the  separation  of  Th  B,  C, 
and  D  from  Th  X. 


4  a-RAY   ACTIVITIES 

as  to  be  almost  invisible;   the  differential  absorption  of  a-rays  in  such 
films  was  therefore  negligibly  small. 

The  time  of  precipitation  of  the  sulphate  was  taken  as  zero  time,  as 
at  this  moment  the  precipitate  was  free  from  all  of  the  subsequent 
products  of  radium.  That  the  foregoing  method  completely  freed  the 
radium  from  all  of  its  active  products  was  shown  both  by  control  experi- 
ments and  by  the  fact  that  the  activity  increased  at  a  regular  rate  from 
the  start.  About  thirty  minutes  elapsed  between  the  time  when  the 
sulphates  were  precipitated  and  the  time  of  making  the  first  measure- 
ments. The  activity  measurements  were  made  in  a  gold-leaf  electro- 
scope, as  previously  described,1  the  active  films  being  placed  7  cm.  below 
the  charged  electrode,  thus  allowing  all  rays  to  reach  their  full  ranges. 
Sufficient  potential  (about  600  volts)  was  used  to  insure  practically 
complete  saturation  currents  for  the  weak  ionization  produced.  All 
activities  were  measured  in  comparison  with  a  standard  film  of  uranium 
oxide,  the  activities  given  being  in  terms  of  this  standard.  All  measure- 
ments were  made  with  the  greatest  care,  and  corrections  were  made  for 
the  accurately  determined  natural  leak,  which  in  every  case  was  less  than 
i  per  cent  of  the  standard.  The  initial  activity  of  each  film  was  less  than, 
the  final  activity  greater  than,  the  standard.  The  activity  increased 
nearly  linearly  for  the  first  six  hours,  so  that  by  a  small  extrapolation 
the  activity  at  time  zero  could  be  determined  with  a  high  degree  of  accu- 
racy. The  activities  in  terms  of  the  standard  are  given  in  Table  II  for 
two  films. 

TABLE  II 
THE  INITIAL  AND  FINAL  ACTIVITIES  OF  RADIUM  (UNCORRECTED) 


Film 

Initial  Activity 

Final  Activity  8 

Ratio  Uncorrected 

I  

0.858 

4.  -12< 

c  .043 

2 

O   S^S 

2    8^2 

ir   Q71? 

But  these  results  are  subject  to  three  corrections,  viz.:  the  activity 
due  to  the  /3-rays,  the  escape  of  emanation  from  the  film  either  by 
diffusion  or  by  recoil,  and  the  loss  of  other  products  by  recoil. 

The  activity  due  to  /3-rays  was  determined  by  placing  consecutive 
layers  of  aluminium  foil  over  the  Ra-BaSO4  film,  the  first  layer  being  of 
sufficient  thickness  to  absorb  all  a-rays  and  measuring  the  activity 
with  each  additional  layer  in  the  same  electroscope  as  was  used  for  the 

1  McCoy  and  Ashman,  Am.  Jour.  Sci.,  26,  521,  1908. 


i< 


FTE: 


a-RAY   ACTIVITIES   AND   RANGES   OF   RADIUM  5 

a-ray  measurements.  By  plotting  the  activities  against  the  thickness 
of  covering,  and  extrapolating  to  the  axis  of  the  activities,  the  /3-ray 
activity  could  be  ascertained  with  a  fair  degree  of  accuracy.  This 
activity  must  be  subtracted  from  the  final  activity  of  the  film. 

Since  Kolowrat1  has  shown  that  the  /3-ray  activity  of  radium  free 
from  its  products  is  only  2  per  cent  of  the  /3-ray  activity  of  radium  in 
equilibrium  with  its  short-lived  products,  and  since  the  /3-ray  activity  of 
the  latter  as  measured  in  the  electroscope  used  in  this  work  was  only 
about  o .  5  per  cent  of  the  a-ray  activity,  it  follows  that  it  is  not  neces- 
sary to  apply  any  correction  for  /3-ray  activity  of  the  radium  itself. 

The  amount  of  emanation  lost  by  the  film  was  determined  by  the 
use  of  a  circular  brass  box,  as  shown  in  Fig.  i.     The  cover  A,  fitted  with 
two  small  bore  tubes  E  and  G,  was  ground  so  as  to  fit  very  tightly  over 
the  lower  compartment  B.     In  the  lower 
compartment  was  a  recess  about  o .  2  cm. 
in  depth  and  8  cm.  in  diameter.     Small- 
bore glass  tubes  drawn  out  at  one  end,  C 
and  D,  were  attached  to  the  tubes  E  and 
G  by  means  of  short  pieces  of  thick-walled 
rubber  tubing.     The  Ra-BaSO4  film  was 
placed  in  B,  the  cover  A  fitted  on,  and 
all  joints  sealed  with  wax.     The  ends  of  C  Fig.  i 

and  D  were  then  sealed  by  fusion.    After 

an  interval  of  40  days  or  more  the  emanation  which  had  accumulated 
was  drawn  into  an  emanation  electroscope  by  a  stream  of  air,  care 
being  taken  to  prevent  any  loss  of  the  accumulated  emanation  and  also 
to  prevent  any  appreciable  diminution  of  pressure  within  the  box  which 
might  withdraw  some  of  the  Em  from  the  film.  The  rate  of  discharge 
of  the  electroscope  was  measured  after  the  emanation  had  stood  in  it 
for  three  hours. 

In  order  to  find  what  fraction  this  quantity  of  Em  was  of  the  equi- 
librium quantity  in  the  Ra-BaSO4  film,  two  additional  determinations 
were  necessary.  One  c.c.  of  the  original  radium  solution  was  diluted  and 
then  freed  from  subsequent  active  products,  as  described  in  a  preceding 
paragraph.  By  means  of  a  small  weight-pipette,  an  accurately  weighed 
portion  of  this  solution  was  taken  and  uniformly  distributed  over  the 
surface  of  a  fiat  platinum  plate  and  evaporated  to  dryness.  The  activity 
of  the  residue,  which  was  entirely  invisible,  was  measured  within  twenty 
minutes,  and  the  measurements  extended  over  a  short  interval  in  order 

1  Le  Radium,  7,  269,  1910. 


6  a-RAY   ACTIVITIES 

to  be  able  to  get  the  initial  activity  by  extrapolation.  To  find  the  activ- 
ity of  the  emanation  from  this  quantity  of  radium,  a  known  portion  of 
the  solution  in  the  weight-pipette  was  run  into  a  small  round-bottom 
flask  containing  about  10  c.c.  of  dilute  hydrochloric  acid.  The  flask 
was  well  stopped  with  a  two-hole  rubber  stopper  fitted  with  delivering 
tubes,  one  of  which  extended  into  the  solution.  A  current  of  air  was 
bubbled  through  the  solution  for  30  minutes  to  drive  off  the  accumulated 
emanation,  and  the  flask  was  then  sealed  by  fusing  the  ends  of  the 
delivery  tubes.  After  an  interval  of  a  few  days  the  emanation  was  drawn 
into  the  emanation  electroscope  by  allowing  air  to  bubble  through  the 
solution,  which  was  heated  near  its  boiling-point.  After  three  hours  the 
rate  of  discharge  of  the  electroscope  was  measured. 

The  corrections  for  loss  of  emanation  by  the  film  were  made  as 
follows: 

Initial  activity  of  film  No.  i o. 858 

Initial  activity  of  Ra  on  the  platinum  plate 660 

The  radium  on  the  platinum  plate  was  obtained  from  i.4942gm.  of 
solution.  After  3  days  21.5  hours  1.5738  gm.  of  the  same  solution 
gave  a  quantity  of  Em  which  discharged  the  electroscope  in  46.45  sec- 
onds. Since  for  3  days  21.5  hours,  i  —  e~™= 0.5036,  the  equilibrium 
amount  of  Em  from  the  solution  would  have  discharged  the  electroscope 
in  o .  5036  X 46 . 45  =  23 . 40  seconds.  Therefore  the  equilibrium  amount  of 
Em  from  i  .4942  gm.  of  this  solution  would  discharge  the  electroscope  in 
24 . 64  seconds.  That  is,  a  radium  film,  free  from  all  subsequent  products 
of  radium,  having  an  initial  a-ray  activity  of  o .  660,  produces  an  equi- 
librium quantity  of  emanation  which  discharges  the  emanation  electro- 
scope in  24 . 64  seconds.  It  then  follows  that  the  equilibrium  quantity  of 
emanation  from  film  No.  i,  which  has  an  initial  activity  of  o  .858,  would 
discharge  the  emanation  electroscope  in  18 .97  seconds.  The  emanation 
which  had  escaped  from  film  No.  i,  when  the  latter  had  been  sealed  up 
40  days  or  more,  discharged  the  emanation  electroscope  in  1,748  seconds; 
therefore  i  .08  per  cent  of  the  equilibrium  quantity  of  Em  escaped  from 
the  film.  This  means  that  the  quantity  of  emanation  and  the  short- 
lived products  in  the  film  is  i  .08  per  cent  too  low.  In  this  film  the  activ- 
ity of  the  subsequent  products  of  Ra  is  4.297  —  0.858  =  3.439,  which 
value  is  i .  08  per  cent  too  low.  Hence  the  true  activity  of  the  products, 
if  no  Em  had  escaped,  would  be  3  .476. 

To  find  the  loss  of  activity  due  to  recoil,  a  polished  brass  plate  7  cm. 
in  diameter  was  placed  i  mm.  above,  and  completely  insulated  from, 
film  No.  i.  A  potential  of  no  volts  was  maintained  for  six  weeks,  the 


a-RAY  ACTIVITIES   AND  RANGES   OF   RADIUM 


upper  plate  being  kept  negatively  charged.  The  activity  of  the  plate 
was  measured  as  quickly  as  possible  after  removing  the  potential,  four 
minutes  elapsing  from  the  time  the  potential  was  removed  to  the  mean 
time  of  making  the  measurements.  The  activity  of  the  active  matter 
which  collected  on  the  plate  was  o .  0374.  Assuming  that  the  active  matter 
on  the  plate  at  the  instant  the  potential  was  removed  was  Ra^4 ,  Ra£, 
and  RaC  in  equilibrium,  the  activity  of  the  matter  on  this  plate  four 
minute's  after  the  potential  is  removed  is  approximately  70  per  cent  of  its 
initial  value.1  Therefore  the  initial  activity  of  the  matter  on  the  plate 
was  o  .0534.  To  find  what  quantity  of  the  matter  on  the  plate  is  due  to 
active  deposit  from  escaped  emanation  and  what  quantity  is  due  to 

TABLE  III 
THE  INITIAL  AND  FINAL  ACTIVITIES  OF  RA  (CORRECTED) 


Film 

Initial  Activity  I0 

Final  Activity  I» 

I./I 

I 

o  8^8 

4.   363 

r    08"? 

2 

o  tx8 

2   86^ 

r    J77 

Mean 

r    IOQ 

direct  recoil,  it  is  necessary  to  recall  that  the  emanation  lost  by  this  film 
was  i .  08  per  cent  of  the  equilibrium  quantity,  or  the  activity  of  the  prod- 
ucts Em,  Ra^4,  and  RaC  lost  was  0.037.  Assuming  that  the  activities 
are  proportional  to  the  two- thirds  powers  of  the  ranges,  the  fraction 
of  the  activity  due  to  Ra^-fRaC  is  65  per  cent  of  the  total  activity  due 
to  Em+Ra^4+RaC,  or  0.0241,  and  that  due  to  recoil  is  0.0534— 
o. 0241  =  0. 0293^ 

Table  III  gives  the  results  of  two  determinations.  The  final  activ- 
ity is  corrected  for  /3-ray  activity,  loss  of  emanation,  and  loss  by  recoil, 
these  corrections  being  determined  separately  for  each  film. 

1  Rutherford,  Radioactive  Substances  and  Their  Radiations  (1913),  p.  491. 

2  It  must  be  pointed  out  that  this  value  is  calculated  on  the  assumption  that  at 
the  instant  the  potential  is  removed  the  active  matter  on  the  plate  is  Ra.4  -f-Ra£+RaC 
in  equilibrium,  and  there  is  a  decided  drop  in  activity  before  the  first  measurement  is 
made,  due  to  the  rapid  decay  of  Ra^4 .     In  the  case  of  the  active  matter  due  to  active 
deposit  we  do  have  this  equilibrium,  but  in  the  case  of  recoil  atoms  there  is  in  all 
probability  an  excess  of  RaC  atoms  over  the  equilibrium  number  of  Ra^4  atoms.    This 
would  mean  that  the  percentage  of  the  activity  which  decayed  during  the  interval 
between  the  time  the  potential  was  removed  and  the  time  of  measurement  would 
not  be  so  great  as  that  calculated  upon  complete  equilibrium,  and  the  value  of  the  loss 
by  recoil  would  be  even  less  than  that  calculated  above.     However,  the  loss  by  recoil 
is  so  small  that  the  value  calculated  above  is  certainly  correct  within  experimental 
error,  and  it  represents  the  maximum  value. 


a-RAY   ACTIVITIES 


In  the  series  Ra—  Em—  Ra^4  —  RaC  it  is  very  probable  that  each 
member  when  present  in  equilibrium  amount  produces  the  same  number 
of  a-particles  per  unit  time;  if  this  is  so,  the  equation 


leads  to  the  theoretical  results  shown  in  Table  IV. 

TABLE  IV 


RANGES 

pi 

RELATIVE  A 

CTIVITIES 

AT  15° 

Calculated 

Found  , 

Ra 

•3      -2Q 

2    217 

I   OO 

(i  oo) 

Em 

3-ov 

A      10 

2    586 

I    17) 

Ra4  

A    7^ 

2    826 

I    28  r4  OO 

4n 

RaC 

6  04. 

•2  6^0 

i  64 

The  only  previous  determination  of  the  relative  activities  of  radium 
and  its  products  was  made  by  Boltwood.1  In  these  experiments  the 
films  were  made  by  the  evaporation  of  a  chloride  solution.  Since  such 
films  gave  off  very  large  fractions  of  the  emanation  produced  (in  one 
case  as  high  as  65  per  cent),  it  is  not  surprising  that  the  results  obtained 
were  not  very  accurate.  Instead  of  a  ratio  of  4.11  given  here  (last 
column,  Table  IV),  Boltwood  found  4.65.  For  several  years  this  result 
was  looked  upon  as  satisfactory,2  since  it  was  in  good  agreement  with 
that  calculated  upon  the  assumption  that  the  number  of  ions  produced 
by  an  a-particle  is  proportional  to  the  first  power  of  its  range3  instead 
of  the  two-thirds  power. 

The  good  agreement  of  this  result  with  the  theoretical  value  as 
calculated  by  the  equation  I  =  kR*,  the  correctness  of  which  may  be 
considered  as  already  established,  shows  that  only  those  members  which 
are  here  considered  play  significant  parts  in  the  a-ray  activity  of  the 
radium  series,  and  that  the  accepted  ranges  are  at  least  approximately 
correct. 

1  Phys.  Zeit.,  7,  489,  1906;  Le  Radium,  3,  170,  1906. 

2  Rutherford,  Radioactive  Substances  (1913),  p.  447- 

3  Boltwood,  he.  cit. 


CHAPTER  II 


THE  RELATION  BETWEEN  a-RAY  ACTIVITIES  AND  RANGES  OF 
RADIOACTINIUM  AND  ITS  SUBSEQUENT  PRODUCTS 

A  study  of  the  actinium  series  was  made  along  the  same  lines  as  in 
the  preceding  part  of  this  paper,  and  radioactinium,  free  from  actinium 
itself  as  well  as  actinium  Xand  subsequent  products,  was  prepared,  thin 
films  made,  and  the  a-ray  activities  measured  from  time  to  time.  The 
activities,  of  course,  increase  to  a  certain  interval,  then  decrease,  owing 
to  the  formation  of  actinium  X  and  its  products  and  the  consequent 
decay  of  the  whole. 

Table  V  gives  the  latest  available  data  for  this  series. 

TABLE  V 

THE  ACTINIUM  SERIES 


Symbols 

Periods 

Rays 

Ranges 
(Centimeters) 

Actinium  

Ac 

? 

Radioactinium 

Rn 

i  8  88    days* 

a   B 

A    4.ot 

Actinium  X   . 

AcX 

ii  ^    days* 

a 

4  4.O 

Emanation 

Em 

3  9      seconds 

a 

£    70 

Actinium  A    .  , 

A 

0.002  seconds 

a 

6  <o 

Actinium  B  

B 

36          minutes 

/3 

Actinium  C  

C 

2  .  i      minutes 

a 

$  .40 

Actinium  D 

D 

4  71    minutes 

B   7 

*This  paper.  The  previously  accepted  periods  of  radioactinium  and  actinium  X  were  19.5  days 
and  10 . 2  days  respectively. 

t  The  range  of  radioactinium  found  by  Geiger  was  4.60  cm.;  the  value  given  above  is  that  given 
in  this  paper. 

If  /i  is  the  fraction  of  the  original  Rn  left  after  an  interval  of  /  days 
and /,. is  the  fraction  for  AcZ,  unit  amount  being  the  equilibrium  quantity 
corresponding  to  the  initial  amount  of  Rn,  then  the  activity  A ,  at  time  /, 
resulting  from  unit  initial  quantity  of  Rn,  is  given  by  the  equation: 


(i) 


where  x  is  the  a-ray  activity  of  the  equilibrium  quantity  of  AcX  plus 
products  for  unit  quantity  of  Rn.  Strictly  speaking,  for  intervals  of  less 
than  3  or  4  days  one  should  take  into  account  the  fact  that  the  products 


10  a-RAY   ACTIVITIES 

of  AcX  are  not  in  equilibrium  with  the  latter;  practically,  however,  for 
longer  intervals  the  simple  equation  (i)  is  entirely  sufficient.  If  Xi 
and  X2  are  the  decay  constants  of  Rn  and  AcX,  respectively,  then 


and 


The  periods  being  18.8  days  and  11.35  days,  respectively,  Xi=  0.0369 
and  X2  =  o.o6n. 

The  actinium  used  in  this  work  had  been  found  to  be  free  from  all 
but  negligible  traces  of  radium  and  thorium  and  their  active  products, 
emanation  tests  proving  the  absence  of  radium  and  active  deposit 
experiments  proving  that  of  thorium.  It  contained,  however,  an  appre- 
ciable amount  of  ionium.  To  the  solution  of  the  actinium-bearing 
material  in  dilute  hydrochloric  acid  a  few  milligrams  of  thorium  nitrate 
was  added  and  the  latter  then  precipitated  with  hydrogen  peroxide. 
The  filtrate  contained  all  the  actinium  and  was  apparently  free  from 
Rn  and  lo;  but,  to  be  certain  that  no  trace  of  the  latter  remained  in  the 
Ac,  the  treatment  with  Th  and  hydrogen  peroxide  was  repeated  three 
times.  To  the  purified  actinium  solution  i  c.c.  of  5  per  cent  aluminium 
chloride  solution  was  added  and  ammonia  gas  (free  from  carbon  dioxide) 
passed  in.  The  aluminium  hydroxide  was  filtered  out,  dissolved  in 
hydrochloric  acid,  and  precipitated  with  ammonia  once  more.  The 
filtrate  from  the  first  aluminium  precipitate  contained  two-thirds  of  the 
AcX;  that  from  the  second  the  larger  part  of  the  balance;  while  6  per 
cent  of  the  AcX  remained  in  the  third  aluminium  precipitate,  which  con- 
tained practically  all  of  the  Ac.  The  precipitations  with  ammonia  had 
freed  the  Ac  from  the  minute  amount  of  mesothorium  introduced  with  the 
few  milligrams  of  Th  used.  This  Ac  solution  stood  for  several  months, 
during  which  time  a  large  amount  of  Rn  formed.  The  Rn  was  then 
separated  from  this  solution,  which  had  a  volume  of  5  c.c.  by  adding 
3  drops  of  a  2  per  cent  solution  of  thorium  nitrate,  which  had  just  been 
freed  from  mesothorium,  and  precipitating  the  Th  by  hydrogen  peroxide. 
The  precipitate  was  filtered  out,  dissolved  in  dilute  hydrochloric  acid 
and  potassium  iodide,  and  reprecipitated  by  hydrogen  peroxide  four 
times  more.  The  10  c.c.  of  acid  solution  of  the  last  Th  precipitate  which 
contained  all  the  Rn,  but  was  entirely  free  from  AcX  and  its  products,  was 
treated  with  oxalic  acid.  The  precipitate  of  3  or  4  mg.  of  thorium 


a-RAY   ACTIVITIES    AND   RANGES   OF    RADIOACTINIUM 


II 


oxalate  contained  all  of  the  Rn.  It  was  separated  from  the  solution  by 
a  centrifuge  and  washed  thoroughly  with  water  and  then  with  alcohol. 
A  small  portion  was  then  spread  on  a  metal  plate  in  an  exceedingly  thin 
film,  the  activity  measurements  of  which  are  given  in  Table  VI.  In  this 
table  x  is  the  activity  of  AcX+Em+A+B+C+D  in  terms  of  the 
activity  of  the  equilibrium  amount  of  Rn  as  unity. 


TABLE  VI. 

THE  a-RAY  ACTIVITY  CURVE  OF  RADIOACTINIUM  INITIALLY 
FREE  FROM  ITS  PRODUCTS.     FILM  No.  4 


Interval  in  Days 

Activity 

X 

o  ooo   

I    OOO 

<?    IOI     . 

I    CH3 

A     ZA 

6.O2^  .  . 

2  .  O4< 

A      C5 

I1?.  76.  . 

2  .  5Q7 

4   cc 

16.08  

2.62O 

4.  en 

17.07  

2.623 

4-  ^0 

17.80  

/- 

2.620 

4.60 

20  o< 

2    ^Q3 

4  <?6 

2-1    10 

2    ^27 

A      CA 

27   OQf 

2.  3Q7 

A      CA 

31    IO 

2.  28l 

4  63 

36   OA 

2  .O1?'? 

4.60 

7Q     Q7      . 

1.881 

4.61 

2O6  .O  

0.007 

Mean 

A      £72 

The  activity  at  the  end  of  206  days  (Table  VI)  was  found  to  be 
0.007;  °f  this,  o.ooi  was  calculated  to  be  due  to  remaining  Rn  and 
products;  the  balance,  0.006,  represents  the  constant  activity  of  the 
minute  amount  of  thorium  present.  The  activities  given  in  Table  VI 
have  been  corrected  for  this  constant  thorium  activity.  The  activity 
measurements  were  made  in  the  a-ray  electroscope  previously  described.1 
The  active  films  were  placed  6 . 5  cm.  below  the  charged  electrode,  thus 
allowing  all  rays  to  reach  their  full  ranges.  The  activities  of  the  films 
varied  between  one-half  and  one  and  one-half  that  of  the  standard 
film  of  uranium  oxide.  Sufficient  potential,  500  to  600  volts,  was  used 
to  insure  practically  complete  saturation  currents  for  the  weak  ionization 
measured. 

Three  other  series  of  measurements  like  those  represented  by  Table  II 
were  made  with  films  made  from  two  additional  lots  of  radioactinium, 

1  McCoy  and  Ashman,  loc.  cit. 


12  a-RAY    ACTIVITIES 

each  prepared  and  purified  separately.     The  results  for  the  four  films  are 
summarized  in  Table  VII. 

The  value  of  x  thus  found  is  subject  to  a  correction,  which  was 
estimated  to  be  2.5  per  cent,  due  to  the  effects  of  /3-rays,  recoil,  and  loss 
of  emanation.  The  corrected  value  is  4.68.  By  direct  measurement 
it  was  found  that  the  /3-rays,  largely  from  actinium  D,  increased  the 
value  of  x,  0.5  per  cent  for  the  a-ray  electroscope  used  in  our  measure- 
ments. To  find  the  loss  of  activity  due  to  recoil  and  loss  of  emanation 
a  brass  plate  charged  to  a  negative  potential  of  no  volts  was  placed 
i  mm.  above  the  radioactive  film  in  a  closed  vessel,  and  the  activity 
which  deposited  on  the  plate  during  the  first  16  days  following  the  prepa- 
ration of  the  radioactinium  film  was  measured.  Almost  exactly  half 
of  the  activity  of  the  deposit  was  due  to  AcC;  the  balance  decayed  with 
the  period  of  AcX,  the  presence  of  the  latter  substance  being  due  to 

TABLE  VII 


No.  of  Film 

No.  of  Observations 

X 

I 

18 

A    en 

2 

14. 

A.  f"7 

•2    . 

1C 

A.  ff 

4-.  . 

12 

4-57 

Mean  

4-57 

recoil.  Assuming  that  all  of  the  AcC  lost  by  the  film  was  deposited  on 
the  plate,  the  correction  to  x  on  this  account  would  amount  to  2  per  cent. 
The  correction  to  AcX,  which  is  accompanied  of  course  by  Em  and  A, 
is  also  apparently  2  per  cent,  but  should  be  taken  somewhat  less  by  reason 
of  the  probable  extensive  escape  in  the  ionization  chamber  of  the  Em  and 
A  from  the  surface  of  the  brass  plate  into  the  space  above,  where  they 
would  cause  greater  ionization  than  if  they  remained  wholly  on  the  plate. 
The  true  correction,  however,  was  taken  to  be  1.8  per  cent  instead  of 
2  per  cent.  A  further  correction  should  be  made  because  of  the  volatili- 
zation of  Em  and  therefore  of  A  from  the  principal  film  in  the  ionization 
chamber  of  the  electroscope.  From  the  amount  of  C  in  the  active 
deposit,  we  estimate  that  2  per  cent  of  the  whole  Em  is  present  in  gas- 
eous form  in  the  electroscope,  and  that  this  would  cause  the  value  of  x 
as  found  to  be  0.8  per  cent  too  high.  The  combined  effect  of  these 
corrections  is  2+1.8—0.5  —  0.8=2.5  per  cent.  Taking  x  uncorrected 
as  4 . 57,  the  corrected  value  becomes  4 . 68. 


a-RAY   ACTIVITIES    AND   RANGES    OF   RADIOACTINIUM  13 

Table  VIII  shows  the  ranges,  R,  at  15°  and  76  cm.,  R*,  and  the  per- 
centage activity  of  each  a-ray  member  of  the  series;  these  percentages 
are  taken  proportional  to  the  corresponding  values  of  R*}  it  being 
assumed  that  the  equilibrium  amount  of  each  member  of  the  series  pro- 
duces the  same  number  of  a-par tides  per  second.1  The  values  given  in 
the  last  column  of  Table  VIII  are  those  found  by  experiment :  for  radio- 
actinium  the  "activity  found"  is  i/(i+#)=  17  .6  per  cent  of  the  whole. 


TABLE  VIII 


RANGES  AT  15° 

V 

PERCENTAGE  ACTIVITY 

Calculated 

Found 

Radioactinium  

4.40 
4.40 
S-70 
6.50 
5-40 

2.69 
2.69 
3-19 
3-49 
3-o8 

17-8 
I7.8 
21.  1 
23.0 
20.3 

82.2 

I7.6 
82.4 

Actinium  X 

Emanation 

Actinium  A  

Actinium  C  

IOO.O 

The  value  of  2R*  for  AcX+Em+A+C  divided  by  R*  for  Rn  gives  4 .62, 
which  is  therefore  the  theoretical  value  of  x.  The  close  agreement  of 
this  result  with  that  found  by  experiment,  4.68,  is  further  evidence,  if 
such  were  necessary,  that  the  roll  of  the  a-ray  members  of  the  actinium 
series  is  now  complete  and  that  the  constants  given  in  Tables  V  and 
VIII  may  be  accepted  with  considerable  confidence  as  being  at  least 
close  approximates  to  the  true  values. 

1  While  it  is  probable  that  AcC  is  complex,  the  number  of  a-particles  of  longer 
range  than  5. 4  cm.  compared  with  those  of  this  range  is  so  small — 0.15  per  cent — 
that  we  need  not  take  them  into  account  here.  See  Marsden  and  Perkins,  Phil.  Mag., 
27,  700,  1914. 


CHAPTER  III 
NOTES 

NOTE   I.      THE  PERIOD  OF   RADIOACTINIUM 

Hahn's  early  work1  on  radioactinium  indicated  a  period  of  19 . 5  days. 
This  value  has  been  accepted  during  the  past  eight  years  and  was  thought 
to  be  confirmed  by  Rothenbach,2  working  under  the  direction  of 
Professor  Hahn.  In  determining  the  period  of  a  radioactive  substance, 
which  produces  products  of  considerably  shorter  life,  it  is  usually  prac- 
ticable to  consider  that  the  rate  of  decay  is  exponential  after  a  sufficient 
lapse  of  time.  It  was  by  this  method  of  calculation  that  the  period  of 
radioactinium  was  determined  by  Hahn  and  by  Rothenbach.  However, 
in  case  the  period  of  one  or  more  of  the  products  is  the  same  order  of 
magnitude  as  that  of  the  mother-substance,  this  simple  treatment  of 
the  problem  is  not  accurate,  and  it  becomes  necessary  to  make  the  follow- 
ing modification: 

The  familiar  equation  which  has  been  previously  used  in  the  form 


may  be  written 


A.-Ai711 

if  for  i  —  e~(A'-A')'  WQ  write  p.     The  equation 


then  becomes 


Now  the  limiting  value  of  /2  is  given  by 


1  Phil.  Mag.,  12,  244,  1906;   13,  165,  1907. 

2  Dissertation  (Berlin,  1913). 

14 


NOTES  15 

and  to  calculate  the  period  of  Rn  by  the  simple  exponential  equation  is 
to  assume  that  /2  has  approached  this  limitating  value  sufficiently 
closely.  But  this  is  by  no  means  the  case,  even  after  a  much  longer  time 
than  that  over  which  Hahn's  and  Rothenbach's  observations  were  car- 
ried. After  115  days  the  activity  of  a  Rn  film  is  about  6  per  cent  less 
than  it  would  be  if  the  limiting  ratio  of  AcX  were  present.  Let  us  repre- 
sent by  A'  the  activity  of  the  film  if  this  limiting  ratio  of  AcX  (+  prod- 
ucts) were  present:  then 


and 

A'      X2-; 


or  we  may  write 

A'  =  Aq. 

In  order  to  find  the  period  of  Rn,  films  were  made  as  previously 
described,  and  very  careful  measurements  were  made  of  their  initial 
activity,  extrapolating  from  the  first  measurements  made  45  minutes 
after  the  precipitation  of  the  Rn  with  the  thorium  oxalate  to  find  the 
true  activity  at  time  zero,  which  value  was  used  in  check  calculations. 
These  films  were  allowed  to  decay  a  suitable  length  of  time,  and  a 
series  of  measurements  was  made  on  each  at  convenient  intervals.  The 
films  were  then  allowed  to  stand  for  nearly  a  year,  and  the  activities 
again  measured  over  a  short  period.  This  activity,  which  was  in  each 
case  nearly  constant,  was  due  to  the  small  amount  of  thorium  present, 
all  Rn  having  decayed.  All  measurements  on  each  film  were  then 
respectively  corrected. 

Table  IX  gives  the  results  of  a  series  of  measurements  made  on 
one  of  the  films,  calculated  on  the  basis  that  #=4.68. 

Three  other  series  of  measurements  of  different  films  gave  the  results 
found  in  Table  X.  The  low  result,  18.75  days,  for  the  last  series  was 
caused  by  three  very  low  values,  found  on  successive  measurements  on 
the  ninth,  twelfth,  and  fourteenth  days,  viz.,  18.56,  18.57,  and  18.67. 
On  both  the  fifteenth  and  sixteenth  days,  however,  the  measurements 
gave  18.84  days,  and  it  is  probable  that  these  low  results  were  caused 
by  some  undiscovered  error  in  the  activity  measurements.  If  these 
low  results  are  omitted  from  the  mean,  the  figure  for  this  series  becomes 


i6 


a-RAY   ACTIVITIES 


18.87  days,  a  value  in  good  agreement  with  the  means  of  the  other 
three  series. 

Further  check  calculations  were  made  of  the  activities  at  various 
times  upon  the  assumption  that  the  period  of  Rn  is  19  .o  and  19 .  i  days. 
If  the  period  of  Rn  is  19 .  i  days,  the  mean  difference  between  the  activ- 
ities found  and  calculated  is  5  .9  per  cent;  for  a  period  of  19  .o  days  the 
difference  is  3  .7  per  cent,  while  for  a  period  of  18 .8  days  the  difference 
is  only  o .  8  per  cent.  It  is  therefore  perfectly  clear  that  the  old  value, 
19.5  days,  is  far  from  the  true  value  and  can  no  longer  be  supported. 

TABLE  IX 


Age  of  Film 
in  Days 

Interval  since 
92.21  Day 

Activity 
Corrected 

Q 

A' 

Period 

QI  .  21  .  . 

O.O 

A 
1.658 

no 

I   840 

IOO.Q7  .  . 

8.76 

1  .227 

.087 

I    734. 

18  86 

103  08 

II    77 

i  106 

080 

I    IOC 

18  88 

1  08   20 

It   00 

QC2 

O72 

I    O2I 

18  81 

112.92  

117-89  

119  16    ... 

20.71 
25.68 
26    Q<? 

•yo-6 
.809 
.678 
652 

.064 
.056 
OCA 

.861 
.717 
687 

18.89 
18.88 

l8  Q7 

Mean  

18  88 

TABLE  X 


Age  of  Film  at  First 
Measurement 

Age  of  Film  at  Last 
Measurement 

Number  of 
Measurements 

Period 

60,  i  days 
60  .  i  days 
104.0  days 

109  .  2  days 
118.0  days 
120.0  days 

II 
12 

7 

18.88 
18.88 
18-75 

The  difference  between  the  value  18.8  for  the  period  of  radioactinium 
and  that  of  Hahn  and  Rothenbach  is  wholly  due  to  the  fact  that  in  the 
latter  case  the  formula  used  in  the  calculation  was  not  sufficiently 
accurate.  The  experimental  results  given  here  are  in  satisfactory 
agreement  with  theirs  and  indicate  also  a  period  of  19.5  days  if  the  cal- 
culation is  made  by  the  simple  exponential  equation  used  by  Hahn 
and  Rothenbach.  The  latter's  calculations  were  based  on  measure- 
ments made  between  the  one  hundred  and  fifteenth  and  one  hundred  and 
seventy-fourth  day.  Now  a  simple  calculation  will  show  that,  if  the 
period  as  found  by  the  simple  exponential  equation  is  19 .5  days  for  the 
interval  between  120  and  139 .5  days,  then  by  use  of  the  correct  formula 
used  here  the  same  data  show  the  true  period  to  be  18.89  days. 


NOTES  17 

These  measurements  indicate  that  the  period  of  radioactinium  is  a 
little  higher  than  18.8  days,  which  value  was  used  in  finding  the  value 
£  =  4 .68.  If,  as  is  very  probable,  the  period  is  18 .88  days,  the  previous 
experimental  results  give  #  =  4.67;  the  difference  is  not  great,  and  the 
lower  value  of  x  is  in  even  closer  agreement  with  the  value  calculated 
from  the  ranges,  viz.,  4.63. 

NOTE   2.      DETERMINATION  OF  THE  RANGE  OF  RADIOACTINIUM 

From  the  value  found  for  the  activity  of  the  products  of  radioactinium 
compared  with  radioactinium  itself  it  was  suspected  that  either  the 
accepted  range  of  radioactinium  was  too  great  or  that  of  AcX  was  too 
small.  The  values  found  by  Geiger1  for  Rn  and  AcX  at  15°  and  760  mm. 
were  4.60  and  4. 40  cm.,  respectively.  Furthermore,  by  Geiger 's  law 
relating  ranges  and  decay  constants,  the  range  of  Rn  should  be  less  than 
that  of  AcX.  As  it  was  a  comparatively  easy  task  to  determine  the 
range  of  Rn,  this  measurement  was  carried  out  with  the  use  of  a  modifica- 
tion of  Geiger's  spherical  flask  apparatus.  The  flask,  which  was  chosen 
from  a  large  stock,  was  almost  perfectly  spherical,  except  in  the  region 
near  the  neck.  Its  internal  radius  was  6 . 7  cm. 

The  radiothorium  was  prepared  by  the  same  method  as  that  used  in 
preparing  this  substance  for  the  activity  measurements.  A  large  quan- 
tity of  actinium  was  used,  and  but  one  drop  of  a  0.4  per  cent  thorium 
nitrate  solution,  =0.1  mg.,  ThO2.  The  thorium  was  precipitated  three 
times  with  hydrogen  peroxide  and  was  converted  into  oxalate,  which 
was  separated  by  means  of  a  centrifuge  and  washed  with  dilute  hydro- 
chloric acid  and  finally  with  alcohol.  A  small  portion  of  this  oxalate 
was  made  into  a  very  thin  film,  about  i  mm.  in  diameter  on  the  center 
of  the  brass  disk  of  the  range  apparatus.  The  range  measurements  were 
completed  within  two  or  three  hours  from  the  time  of  the  oxalate  pre- 
cipitation; from  this  it  follows  that  the  maximum  amount  of  AcX 
formed  did  not  exceed  0.25  per  cent  of  the  equilibrium  amount.  The 
temperature  was  read  at  frequent  intervals  during  the  measurement  and 
always  remained  constant  within  one  degree. 

The  results  of  one  determination  are  shown  graphically  in  Fig.  2, 
which  is  drawn  to  scale.  The  break  comes  at  504  mm.  of  mercury,  and 
as  the  temperature  was  20°,  this  gives  as  the  range  of  radioactinium 
4.17  cm.  at  760  mm.  and  o°,  or  4 .40  cm.,  at  15°.  Two  other  determina- 
tions were  made,  each  with  separately  prepared  samples  of  radioactinium. 

1  Phil.  Mag.,  24,  653,  1912. 


i8 


a-RAY    ACTIVITIES 


One  gave  4.38  cm.,  the  other  4.43  cm.,  at  15°;  the  mean  of  the  three 
results  is  4. 40  cm.  for  15°.  This  is  0.20  cm.  lower  than  Geiger  found 
for  radioactinium  and  is  identical  with  Geiger's  value  of  actinium  X. 
The  periods  of  radioactinium  and  actinium  X  are  18.8  and  n  .35  days, 
respectively,  and  if  the  range  of  the  former  is  4 . 40  cm.  that  of  the  latter 


1.2 


I  i.o 


.£ 
x 
'> 

1 


300 


400  500 

Pressure  in  mm.  of  mercury 
Fig.  2 


600 


700 


should  by  Geiger's  law  be  4.45  cm.,  which  differs  from  the  value  found 
by  Geiger  by  scarcely  more  than  the  experimental  error  of  the  measure- 
ments. 

NOTE  3.      THE  PERIOD  OF  ACTINIUM  X 

Kolowrat1  gives  in  his  annual  tables  of  radioactive  constants  the 
value  for  the  period  of  actinium  X  as  10  to  n  days.  Goldewski2  has 
previously  reported  this  figure  to  be  10.2  days.  Hahn  and  Rothen- 
bach3  have  recently  given  the  value  to  be  1 1 . 6  =*=  o .  i  days.  In  view 
of  the  uncertainty  of  this  figure,  work  along  this  line  was  undertaken 
to  ascertain  its  true  value. 


1  Le  Radium,  loc.  cit. 

3  Phil.  Mag.,  10,  35,  1905. 


3  Phys.  Zeit.,  14,  409,  1913. 


NOTES  IQ 

Radioactinium  free  from  actinium  itself  and  all  subsequent  products 
was  prepared  as  previously  described,  except  that  the  precipitation  of  the 
thorium  as  oxalate  was  omitted.  The  solution  was  allowed  to  stand  for 
about  two  weeks;  a  portion  of  this  solution  was  then  nearly  neutralized 
with  ammonia  and  the  thorium  precipitated  by  means  of  hydrogen 
peroxide.  The  solution  which  contained  the  actinium  X  was  filtered  and 
10  drops  of  2  per  cent  thorium  nitrate  solution  were  added.  ThO2  was 
again  precipitated  and  the  solution  filtered.  This  treatment  was  repeated 
three  more  times,  insuring  the  complete  removal  of  Rn.  A  portion  of 
the  last  filtrate  was  placed  in  a  15  c.c.  centrifuged  tube,  i  c.c.  of  o.oi  N 
BaCl2  added,  then  a  few  drops  of  dilute  H2SO4.  The  solution  was  allowed 
to  stand  15  minutes  and  then  centrifuged.  The  BaSO4  which  carried 


TABLE  XI 


Time  in  Days 

Activity 

Decay   Constant 

Time  in  Days 

Activity 

Decay  Constant 

O.OO  

1  .000 

O.OO  

I  .OOO 

3  .  06  

0.830 

0.0610 

3  .  71  .  . 

o.  708 

o  0609 

5  .  04  

~J~ 

0.736 

0.0610 

4.71  

o.  751 

O  0609 

6.77  

f 
0.659 

0.0616 

6.72  

0.664 

0.0610 

9  .  83  

0.546 

0.0616 

8.84  

0.581 

0.0614 

II    00 

o  480 

o  0613 

0   72 

O    ZZI 

o  0613 

12    72 

o  4^8 

o  0614 

12   8l 

O  4^7 

o  0611 

Is;  .02  .  . 

0.380 

o  0610 

i  7  .  oo  ...    . 

O   3^7 

o  0607 

19.08  

0.318 

0.0608 

Mean 

o  0612 

Mean 

o  0610 

down  the  actinium  X  was  then  washed  and  made  into  films  as  previously 
described  in  making  the  Ra'BaSO4  films.  Two  films  were  prepared  in 
this  manner.  Each  was  allowed  to  stand  for  two  days  before  the  series 
of  measurements  was  begun.  The  measurements  were  made  in  the 
a-ray  electroscope  used  in  the  previous  work,  the  active  films  being 
placed  7  cm.  below  the  charged  electrode.  All  activities  were  measured 
in  comparison  with  a  standard  film  of  uranium  oxide,  and  corrections 
were  made  in  every  case  for  the  natural  leak.  A  small  correction  was 
applied  for  the  presence  of  thorium  X,  introduced  by  the  thorium 
nitrate  solution.  This  correction  was  obtained  by  running  a  blank 
experiment,  and  in  every  case  it  was  less  than  o .  3  per  cent  of  the  total 
activity. 

Table  XI  gives  the  values  obtained,  the  activities  given  being  cor- 
rected for  all  known  factors. 


20  a-RAY    ACTIVITIES 

The  mean  of  the  values  for  the  decayed  constants  is  therefore 
0.0611  day"1,  corresponding  to  a  period  of  11.35  days.  This  value 
is  in  close  agreement  with  that  of  Hahn  and  Rothenback  and  disproves 
that  given  by  Godlewski. 

In  conclusion  the  author  wishes  to  express  his  most  sincere  thanks 
to  Professor  Herbert  N.  McCoy,  whose  interest,  co-operation,  and 
encouragement  have  made  the  completion  of  this  work  possible. 


Syracuse  N  Y. 
PAT.  JAN.  21,  1908 


404734 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 


